An injection unit of this type and an injection method of this type are known for example from DE 100 15 740 A1. In this known technology, an injector arrangement comprises at least one servo injection valve, which can be actuated by means of a piezoelectric actuator, to cause a displacement of a servo valve nozzle body (nozzle needle) in the direction of an opening of an injection passage which is provided between a nozzle chamber of the servo injection valve and a combustion chamber of the internal combustion engine concerned, for initiating an injection process by pressure reduction in the control chamber.
A key advantage of using a servo injection valve actuated by means of a piezoelectric actuator is that a comparatively small excursion of the piezoelectric actuator can achieve an excursion of the nozzle body that is independent thereof and, as a rule, many times greater (excursion ratio). In addition, the advantage is produced here that the displacement of the nozzle body for opening and closing the injection passage is driven by the pressure of the fuel which for the purposes of injection into the combustion chamber is in any case available under comparatively high pressure in the area of the injection valve. A piezoelectric actuator having a comparatively limited excursion and comparatively low actuating force is therefore adequate for actuating the injection valve.
A piezoelectric actuator comprises, as a rule, a stack of piezoelements lying on top of one another which, when an electric voltage is applied, rapidly alters its length by an extent dependent upon, among other things, the voltage. A great variety of piezoelectric ceramics are known that are suitable for this purpose, for example lead zirconate/titanate ceramics, and are of interest for use in injection valves principally due to their rapid rate of change and their high piezoelectric forces.
Since, however, the length of the piezoelectric actuator does not depend exclusively on the voltage applied, but is also subject, for example, to manufacturing tolerances and a dependency on the temperature of the actuator, when a servo injection valve actuated by a piezoelectric actuator is designed, a more or less large gap is provided in the path of action from the actuator to a control valve body which serves as a range of tolerance for undesired variances and/or changes in the actuator length.
This so-called tolerance gap in the piezo-actuated injection valve should, on the one hand, be dimensioned as small as possible in order to maximize the usable excursion of the actuator, and on the other hand be dimensioned as large as possible in order to avoid, in all operating states if possible, a change in the length of the piezoelectric actuator caused by the operation exceeding the tolerance gap and in this way, without the actuator being actuated, actuating the control valve. Particularly important in the latter regard is, for example, a thermally driven extension of the piezoelectric ceramic at raised actuator temperature, as can occur under certain circumstances, particularly in the operation of the internal combustion engine. Accordingly, the tolerance gap can in practice be difficult to dimension “optimally”.
If the tolerance gap due to a temperature increase of the actuator can be exceeded and if consequently the fuel fed from the pressure reservoir via a pressure line to the control chamber can be released further via the control valve into the practically unpressurized (compared with the fuel system pressure in the pressure reservoir) fuel return line, then further problems arise. Namely, if the internal combustion engine is to be started in a “warm state”, e.g. after the internal combustion engine had previously run for quite a long period and subsequently been switched off, then due to the release of fuel from the control chamber into the fuel return line, the building up of pressure in the pressure reservoir can be hampered or delayed considerably. The build-up of a certain minimum system pressure, which typically stands at a few hundred bar, is however necessary in order to be able to achieve any injection at all from the nozzle chamber into the combustion chamber.
From DE 199 05 340 C2 a method and an arrangement for presetting and dynamically correcting piezoelectric actuators are known in which a direct voltage, possibly superimposed on a pulsed actuation voltage, is fed for this purpose to the piezoelectric actuator. This direct voltage component then determines a new position of rest of the actuator and can thus be used for adjusting the idle stroke and for correcting the idle stroke when running.
From DE 37 42 241 A1, a piezoelectric control valve is known which consists of a piezoelectric actuator arranged in a housing and a valve. Possible changes in length of the reference system are automatically compensated for by a hydraulic play-compensating element inside the control valve, so that for a given working excursion of the piezoelectric actuator a uniform excursion is always ensured at the valve. A disadvantage of these two approaches to solving the problems explained in the introduction is the outlay associated with them in terms of the electronic devices for controlling the injector arrangement and in terms of the injector arrangement itself.